Endless belts are commonly used for applications wherein they are subjected to high stress. In particular, endless metal belts which are used to transmit force in a pulley system such as a continuously-variable transmission (CVT) system or which repeatedly pass over any other sets of rollers such as in a belt-based photocopier are commonly exposed to a high degree of tensile stress and compressive stress. For example, when a belt is flexed, the outside surface of the belt is subjected to tensile stress, while the inside surface of the belt is subjected to compressive stress. Thus, there is a need to design a belt which is sufficiently strong to withstand these tensile and compressive stresses during operation of a system wherein a large number of flexures of the belt occur.
Failure of a member such as a belt occurs if one exceeds the tensile strength or the compressive strength of that member. In particular, a member may fail if the member breaks from a given stress (i.e., exceeding the ultimate tensile strength) such as, for example, 65,000 to 150,000 psi for a nickel belt. A member may also fail if it becomes permanently deformed (i.e., when the yield strength is exceeded). In an endless belt which is flexed, e.g., around rollers, at the point of maximum flexure the belt is subjected to both tensile stress and compressive stress, which leads to relatively rapid failure in a conventional belt. In particular, the radially outer surface of the belt is tensilely stressed, while the radially inner surface of the belt is compressively stressed. This is expressed by the following mathematical relationship: EQU S=(Y.times.w)/r
wherein S is the stress at any point in the belt;
Y is Young's modulus for the belt material;
w is the radial distance from the neutral plane of operational stress (i.e., the radially central plane of the belt) to the point of the belt (i.e., one-half the thickness of the belt) to a point on the outer surface of the belt) where tensile stress is at a maximum; and
r is the radius of the roller.
The value of w is positive moving radially outwardly from the neutral plane, leading to a positive (tensile) S; its value is negative moving radially inward from the neutral plane, leading to a negative (compressive) S.
To design a belt which is not prone to failure, it is necessary to keep the stress which develops in the belt (S) less than the yield stress which is known for that belt material. For example, the tensile yield stress of nickel may range from 50,000 to 85,000 psi. Therefore, the belt should have a maximum tensile stress of less than 50,000 psi in operation. However, the Young's modulus for nickel is 30,000,000, and thus to achieve a maximum tensile stress of, for example, 45,000 psi on the outer surface of a belt 0.003 inches thick, a roller with a 1 inch radius is required. This size roller is not advantageous for many of the intended uses of the belt of this invention. In a CVT application, for example, the belt may be required to carry an additional several thousand psi in use. Even in a copier, photoreceptor belts are typically tensilely stressed to about two thousand psi to insure that they run flat and grip the drive rollers, (e.g., a 0.001 inch thick belt which is 10 inches wide which is carrying a 50 pound load is under 5,000 psi tensile load before it is bent over a roller). These stresses are additive, thus causing one to use either thinner belts or bigger rollers. However, bigger rollers take up more space, weigh more and are more costly. Thinner belts are harder to handle without damage and are limited as to how much they can do. Thus, a method of forming a belt which can be used on a much smaller roller is desirable.
The use of a smaller roller is highly advantageous because it requires less material, weight and space, and thus lends itself to applications wherein miniaturization is desirable. A method of forming a thick belt for use on large radius rollers is also desirable, but such arrangements generally fail because of the large amount of tensile stress in the outer surface of a thick belt. Such a method is particularly useful in the design of photoreceptors which employ self stripping rollers. To self strip paper generally requires a 0.5 inch roller (0.25 radius); most paper will self strip off a 0.75 inch roller. Self-stripping is particularly useful because it eliminates stripper fingers which may cause premature failure of photoreceptors. However, this means that one is required to use belt photoreceptors which are very thin. One can just handle a 0.002 inch thick photoreceptor which is 3.3 inches in diameter, while 0.003 to 0.004 inches in thickness is required for a photoreceptor which is 10 inches in diameter.
U.S. Pat. No. 5,221,458 to Herbert et al. discloses an electroforming process for forming a multilayer endless metal belt assembly which includes forming increasingly compressively stressed successive belts on a mandrel, and assembling the belts to form a multilayer belt assembly. As the belts are removed from the mandrel, the compressive stress is relaxed, creating a precisely controlled gap between adjacent belts. The belt assembly so formed is particularly useful as a driving member for a continuously-variable transmission.
U.S. Pat. No. 4,501,646 to Herbert discloses an electroforming process for forming hollow articles having a small cross-sectional area. This patent discloses an electroformed belt having a thickness of at least about 30 .ANG. and stress-strain hysteresis of at least about 0.00015 in./in., and wherein a tensile stress of between about 40,000 psi and about 80,000 psi is imparted to a previously cooled coating to permanently deform the coating and to render the length of the inner perimeter of the coating incapable of contracting to less than 0.04% greater than the length of the outer perimeter of the core mandrel after cooling. Any suitable metal capable of being deposited by electroforming and having a coefficient of expansion between about 6.times.10.sup.-6 to 10.times.10.sup.-6 in./in./.degree.F. may be used in the process.
U.S. Pat. No. 3,963,587 to Kreckel discloses a method for electroforming relatively smooth seamless nickel, cobalt or nickel-cobalt alloy foil cylinders from an electrolyte for nickel or cobalt, the method comprising slowly increasing the current density from zero to its ultimate current density at the start up of the plating cycle.
U.S. Pat. No. 4,972,204 to Sexton discloses an orifice plate for use in ink jet printing which includes a first elongated lamina composed of electroformed metal or metal-alloy having a tensile or compressive stress condition and a second elongated lamina composed of metal or metal alloy electroformed onto the first lamina and having a counterbalancing stress condition. The electroformed plate has the following characteristics: 1) it operates effectively in longer array formats with planar wave stimulation; 2) it provides a plate construction with an increased thickness while maintaining a high flatness for the array surface; and 3) it has enhanced acoustic stiffness.
When an electroforming process is used to produce compressively stressed belts, they will generally have an inherent increasingly compressive stress gradient. Examples of such uncontrolled internal stress gradients are depicted as curves A and B in the graph shown in FIG. 1, based on a nickel electroforming bath and a chromium mandrel. Curve A depicts the stress gradient formed in a deposit on a normal chromium tank-finished mandrel. Curve B depicts the stress gradient formed in a deposit on a ground-finished mandrel. As shown by the manner in which both curves descend quickly, in electroformed compressive belts of the prior art, the initial deposit is tensilely stressed, but very quickly and uncontrollably becomes compressively stressed. Shortly after the deposit becomes compressively stressed, the internal stress levels off, and the degree of internal compressive stress over time remains fairly constant after the first few minutes.